The invention relates to methods of reducing cancer growth in biological systems. More specifically, the invention relates to the inhibition of solid tumor invasiveness and metastasis in mammals.
Cancer, in all of its myriad manifestations, remains a devastating scourge upon mankind. While progress in preventing and treating cancer has been made, including particular success against Hodgkin's lymphoma and certain other forms, many types of cancer remain substantially impervious to prevailing treatment protocols. Typically, cancer is treated by chemotherapy, in which highly toxic chemicals are given to the patient, or by radiotherapy, in which toxic doses of radiation are directed at the patient. While commonly effective to kill huge numbers of cancer cells, these "cytotoxic" treatments also kill extraordinary numbers of healthy cells, causing the patient to experience acute debilitating symptoms including nausea, diarrhea, hypersensitivity to light, hair loss, etc. The side effects of these cytotoxic compounds limits the frequency and dosage at which they can be administered. Such disabling side effects can be mitigated to some degree by using compounds that selectively target cycling cells, i.e., interfering with DNA replication or other growth processes in cells that are actively reproducing. Since cancer cells are characterized by their extraordinary ability to proliferate, such protocols preferentially kill a larger proportion of cancer cells in comparison to healthy cells, but cytotoxicity and ancillary sickness remains a problem.
Other more recent developments include efforts to develop monoclonal antibodies specific for oncogenes or HLA specificities, to identify cancer cells with great precision. However, these procedures are very expensive and extremely procedurally elaborate, yet still fail to produce the desired efficacy. Indeed, such procedures have been reported to be effective in only a small subpopulation of treated patients.
The area of cancer research concerned with the mechanisms of tumor cell invasion has benefited greatly from the conceptual framework proposed by Liotta and colleagues (see, e.g., Yamamoto et al. (1996); Emmert-Buck et al. (1994). This model describes the invasive process as a logical progression of events involving three discernible stages: attachment of tumor cells to an extracellular matrix (ECM), proteolytic digestion of the matrix, and movement of cells through the proteolytically degraded barrier. A key factor in this process is the regulation of the matrix metalloproteinases (MMPs; including gelatinases A and B; MMP-2 and MMP-9, respectively, and MMP-3 (Lokeshwar et al. 1993a)), that play a major role in the degradation of the ECM during invasion.
Tetracycline and a number of its chemical relatives form a particularly successful class of antibiotics. Certain of the tetracycline compounds, including tetracycline itself, as well as sporocycline, etc., are broad spectrum antibiotics, having utility against a wide variety of bacteria. The parent compound, tetracycline, has the following general structure: ##STR1## The numbering system for the multiple ring nucleus is as follows: ##STR2##
Tetracycline, as well as the 5-OH (terramycin) and 7-Cl (aureomycin) derivatives, exist in nature, and are all well known antibiotics. Semisynthetic derivatives such as 7-dimethylamino-tetracycline (minocycline) and 6.alpha.-deoxy-5-hydroxy-tetracycline (doxycycline) are also known antibiotics. Natural tetracyclines may be modified without losing their antibiotic properties, although certain elements of the structure must be retained to do so. The modifications that may and may not be made to the basic tetracycline structure have been reviewed by Mitscher (1978). According to Mitscher, modification at positions 5-9 of the tetracycline ring system can be made without causing the complete loss of antibiotic properties.
However, changes to the basic structure of the ring system, or replacement of substituents at positions 1-4 or 10-12, generally lead to synthetic tetracyclines with substantially less, or essentially no, antibacterial activity. For example, 4-de(dimethylamino)tetracycline is commonly considered to be a non-antibacterial tetracycline.
More recently, it has been established that tetracyclines, which are rapidly absorbed and have a prolonged plasma half-life, exert biological effects independent of their antimicrobial activity (Golub et al. 1991, Golub et al. 1992, Uitto et al. 1994). Such effects include inhibition of matrix metalloproteinases (abbreviated "MMPs"), including collagenases (MMP-1; MMP-8; MMP-13) and gelatinases (MMP-2; MMP-9), as well as prevention of pathologic tissue destruction (Golub et al. 1991). Recent studies have suggested that, in some systems, certain tetracyclines and inhibitors of metalloproteinases can inhibit tumor progression (DeClerck et al. 1994) or angiogenesis (WIPO publication WO 92/12717; Maragoudakis et al. 1994). Zucker et al. (1985) showed that minocycline can inhibit melanoma cell activity in vitro. Some tetracyclines may exhibit cytotstatic effects against some tumors (Kroon et al. 1984; van den Bogert et al. 1986).
However, the use of tetracycline antibiotics, while generally effective for treating infection, can lead to undesirable side effects. For example, the long term administration of antibiotic tetracyclines can reduce or eliminate healthy microbial flora, such as intestinal flora, and can lead to the production of antibiotic resistant organisms or the overgrowth of yeast and fungi. Accordingly, chemically-modified tetracyclines, in which the antimicrobial activity is attenuated or deleted, can be preferred for use in applications in which anti-collagenolytic activity is indicated.
In view of the above considerations, it is clear that there is a need to supplement existing methods of inhibiting cancer cell invasiveness and metastasis. Current approaches rely on highly cytotoxic compounds that cause ancillary debilitating sickness in patients, or use methodology that is expensive, procedurally difficult, and unpredictable.
Accordingly, it is one of the purposes of this invention to overcome the above limitations in cancer treatment, by providing a compound and method for inhibiting the growth processes characteristic of cancer cells, including inhibiting invasiveness and metastasis, as well as inducing regression of primary tumors. In particular, it is desirable to identify new anticancer compounds and methods that inhibit cancer growth specifically and with relatively high activity, i.e., being active at doses that are substantially free of harmful side effects.